1. Field of the Invention
This invention relates to magnetic resonance (MR) imaging and more specifically to the simultaneous detection of multiple MR images from a patient.
2. Description of Related Art
Several methods for the detection of multiple two-dimensional magnetic resonance images in a single examination are currently in use. These include the acquisition of a three-dimensional matrix of image data which is retrospectively examined by the extraction of arbitrarily oriented two-dimensional slices, or planes, and the interleaved acquisition of arbitrarily oriented spin-echo images as disclosed by Smith et. al. in U.S. Pat. No. 4,871,966 issued Oct. 3, 1989.
Typically, interleaved acquisition of magnetic resonance data is accomplished by the excitation, and detection of magnetic resonance data, from a first arbitrarily oriented slice of a subject, followed by the excitation and detection of a second arbitrarily oriented slice, which in turn is followed by excitation and detection of subsequent slices. During the excitation and detection of each slice, the longitudinal magnetization in each of the remaining slices is allowed to return to its equilibrium value to provide the desired image contrast. In traditional spin-warp implementations of the interleaved acquisition method, the steps of excitation and detection of magnetic resonance data are repeated many times, each time with a unique amplitude of a phase-encoding magnetic field gradient pulse. Fourier transformation of each detected MR data set provides spatial information in a first direction while Fourier transformation of the data with respect to the amplitude of the phase-encoding gradient pulse provides spatial information in a second direction typically orthogonal to the first direction.
One disadvantage of the interleaved data acquisition strategy described above is that periods of time exist during the excitation and detection of data in which no magnetic field gradient activity is present. This is particularly true for image acquisition in which the echo time, TE, is relatively long. An undesirable consequence of these periods of gradient inactivity is that the maximum number of slices which can be detected in a single exam is limited. The number of detectable slices can be determined by dividing the time allowed for the recovery of longitudinal magnetization, TR, by the time it takes to excite and detect data from each slice. This period of time is equal to the echo time, TE, plus the additional time required to fully excite and detect the transverse magnetization.
Presently, there is a need for a more efficient method for the generation of multiple slice images in which the number of detected slices is increased.